Two techniques are currently known to be used for preparing such papers.
The first technique which is known as chromium cylinder coating or "cast coating", consists in feeding the paper web to be treated over a perfectly polished and heated chromium-plated cylinder which said web partly encircles, after the web has been picked up by two reversing rollers.
Just before wrapping around the heated chromium-plated cylinder, the paper web is coated, on its internal face, for example by injection, with a coating slip containing, besides the conventional pigments, special binders and additives which will enable the coating to retain a certain malleability while said coating is in contact with the chromium-plated cylinder and which will make it then easier to detach from the cylinder.
The coating slip is spread out, compressed and heated between the paper and the chromium-plated cylinder, in order to go through, simultaneously to being spread, a sort of simultaneous calendering and drying.
Adequate results may be obtained with feeding speeds ranging between 0 and 50 or 100 m/min. This limited feeding speed requirement corresponds to the necessity to have, on the other side of the cylinder, a paper which is dry enough to be detachable from the chromium-plated cylinder.
The disadvantage of this type of technique is, therefore, a low speed of production which, besides, demands a specific installation to ensure the winding of the paper web, the permanent heating of the chromium-plated cylinder expensive to produce, and the injecting of the coating slip.
A further disadvantage of such a technique is the difficulty in producing a paper exhibiting a high degree of gloss on both faces. Indeed, when the second face is coated, the discharged steam has to go through the paper and the coating of the first coated face. When this steam is discharged, it damages the coating of the first face and makes it impossible to obtain a suitable second face because of a slowing down in the discharge of the steam imposed by the necessity for it to go through the paper and the first applied coating.
Moreover, this technique is also known to be unsuitable for adequately coating a base other than paper, and in particular a plastic material base, due to the non-permeability of such a material.
A second technique consists in applying a coating slip, by the conventional methods, such as with a trailing blade coater, on one face of a base paper, the coating slip being composed so as to contain an adequate quantity of plastic pigments, hereinafter designated also as gloss pigments, and constituted of spherical particles of mean diameter ranging between 0.05 and 1 .mu.m, and being preferably around 0.5 .mu.m.
According to this technique, the coating is applied over a sufficient thickness, generally between 3 and 25 .mu.m, to obtain a coating strong enough to go through a calendering treatment.
After calendering, and as a result of the dispersion of the gloss pigments through the thickness of the coating, an obvious gloss characteristic is noted but this does not reach the target 80%, even with gloss pigment contents reaching up to 30% by weight of pigments conventionally used in the composition of coating slips.
It could be assumed that the way to obtain this characteristic would be in increase in the required proportions, the plastic pigment filler incorporated to the coating slip. Although this step may, in theory, appear obvious, in practice it is not feasible for two reasons.
The first reason is the very high price of the coating slip composition, resulting from the high price of the plastic pigments used therein.
The second reason is that it is found, when taking such a step, that calendering brings about an unexpected side-effect which is harmful. Indeed, as the coating contains a strong proportion of plastic pigments, generally higher than 30% by weight, although it is easily compacted during calendering, it nevertheless suffers a loss of opacity which is harmful in itself, but which is additionally marked by a defective surface commonly known as blackening in the papermaking industry. Such a blackening corresponds to the appearance of a more or less heterogeneous grey color which is the result of a kind of vitrification of the paper. Such a drawback is a serious impediment in the production of high gloss papers of white or pastel color, but even of dark colors.
Therefore, on the whole, the current techniques do not produce a high gloss paper or other base with a degree of gloss at least equal to 80% and in particular, they do not provide a teaching liable to direct the man skilled in the art towards finding a solution to this problem.